stream-clone: introduce the notion of an experimental "v3" version
We introduce a new experimental "v3" stream protocol, disabled by default. In
practice the "v3-exp" protocol introduced in this changeset is identical to v2,
but this changeset, lay the groundwork for having a new protocol:
configuration, capability exchange, test coverage, etc.
The actual protocol work will starts in the coming changesets.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).